Neuroscience has an essential requirement for large-scale neural recording technologies to ensure rapid progress in the understanding of brain function, diagnosis and treatment of neurological disorders. At present, a large gap exists between the localized optical microscopy studies looking at fast neuronal activities at single cell resolution level and the whole-brain observations of slow hemodynamics and brain metabolism provided by the macroscopic imaging modalities. The proposed two-year project is aimed at developing novel optoacoustic neuroimaging tool to volumetrically monitor activity of large distributed neuronal populations with unprecedented temporal resolution in the millisecond range. This goal will be accomplished by constructing a tomographic optoacoustic scanner to simultaneously record three- dimensional optoacoustic data in a spherical geometry. The high temporal resolution in volumetric recordings will make it possible to directly track action potentials using fast voltage-sensitive indicators. The resulting scanner will simultaneously record activity from large fields of view in scattering brains, potentially reaching the mouse hippocampus and beyond. Rapid tuning of the excitation laser wavelength will be further employed to enable simultaneous acquisition of five-dimensional (i.e. real-time volumetric multi- spectral) optoacoustic data, which will provide enhanced sensitivity in detecting rapid spectral variations of the activity reporters. The plan of action includes screening of several potential candidates for voltage imaging, including genetic indicators, using neuronal cell cultures. System validation will be subsequently performed in isolated scattering brains of adult zebrafish and mice, aiming at establishing sensitivity and spatiotemporal resolution metrics in detecting voltage signal transients due to spontaneous and stimulus- driven activity patterns. While the major importance of 3D optical microscopy techniques like two-photon imaging has been highlighted recently within the BRAIN initiative, they are generally limited to looking at small (~1mm3) superficially-located volumes with relatively slow temporal resolution, further requiring highly invasive cranial windows that can alter activity. In contrast, the proposed method is tailored for non-invasive deep brain observations and is ideal for simultaneous imaging of large fields of view at rapid volumetric frame rates and resolution approaching cellular scale. Furthermore, other optoacoustic approaches looked so far only at hemodynamic changes and blood oxygenation, slow and indirect indicators of brain activity. The proposed study will be the first to examine fast optoacoustic signatures of voltage-sensitive indicators, thus shattering the longstanding penetration barrier of optical microscopy in scattering brains.